An intact human foot, connected to its ankle, travels through stance and swing phases of a gait cycle during each stride of motion, whether the motion involves walking, jogging, or running. By adjusting the stiffness and damping characteristics of a prosthetic ankle or foot mechanism, the springiness of the intact natural human foot and its corresponding natural human joints may be mimicked, thereby optimizing the prosthetic for the desired motion of the wearer. The characteristics that are desired to store and release energy appropriately for walking tend to oppose those best suited to fast walking and running.
In a stance phase, the foot is in contact with the ground and the weight of a person is supported on the foot. In a swing phase, the foot is off the ground as the entire leg and foot move from a posterior position to an anterior position with respect to a center of gravity of the person.
The stance phase A, as shown in FIG. 1, begins just after completion of the swing phase and commences with a heel strike wherein the foot is lowered to the ground as the body moves forward from a position posterior to the person's center of gravity. Immediately after heel strike, the foot moves from a dorsi-flexed position, wherein the toes of the foot are pointed upwards, to a plantar-flexed position B wherein the bottom of the foot or shoe is flat on the walking surface, which provides greater stability as the entire weight of the person is shifted over the foot in contact with the ground.
The swing phase C commences just after heel strike of the other foot. During the swing phase, the foot is again in the dorsi-flexed position D as the foot leaves the walking surface and the foot and leg swing forward in preparation for the stance phase. Dorsi-flexion is important for normal human locomotion, since the toes are dorsi-flexed in order to clear the floor. If the foot is not dorsi-flexed during the swing phase, it would most likely catch on the walking surface and cause the person to stumble and fall, which may lead to serious injury.
It is beneficial for the joint mechanism of a prosthetic ankle or foot to have the ability to resist plantar flexion at heel strike and to store energy during dorsi flexion/extension. During the swing phase, lifting the toe is also desirable. If the dynamic response is too stiff, the foot bounces back too quickly. If the spring is too soft, it stores less energy and releases too late.